This invention relates to flexible multilayer polyimide metal-clad laminates comprising at least one layer of an aromatic polyimide and at least one layer of a metallic substrate for use in flexible printed circuits and tape automated bonding applications. The invention also relates to a process for producing these laminates.
Laminates comprising one or more layers of polyimide and one or more layers of metallic substrate material may be used for a variety of applications. For example, polyimide coated metal foils, due to the flexibility and outstanding mechanical, thermal and electrical properties of polyimides, can be used for printed electrical circuits. This is because the laminates are frequently exposed to high temperatures during further processing, for example, during soldering or drilling. The laminates also have to satisfy stringent requirements in regard to their electrical and mechanical properties.
Laminates comprising only one substrate layer of metal or metal alloy and a layer of polyimide, so called single clads, may be used for printed electrical circuits. The same applies to multilayer laminates, so called multi-clads or multilayer circuits, which comprise several metal layers and/or several polyimide layers.
Laminates containing polyimides and metal substrates are well-known in the art. Usually the polyimide layers are bonded to the metal substrate by a conventional adhesive. For example, U.S. Pat. No. 3,900,662 and U.S. Pat. No. 3,822,175 disclose bonding of polyimide to metal using an acrylate-based adhesive. However, it has been found that when conventional adhesives such as acrylates, epoxides, polyamides, phenolic resins etc. are used to bond the polyimide to the metal, the resulting laminates do not exhibit entirely satisfactory properties which meet the stringent demands often imposed. Conventional adhesives do not generally possess the high temperature heat stability of the polyimide material itself, and the strength of the adhesive bonds in multilayer laminar polyimide structures deteriorates rapidly when subjected to elevated temperatures.
On account of the disadvantages of laminates comprising layers of conventional adhesives between polyimide and metal, multilayer laminates have been proposed in which the polyimide is bonded directly to metal, i.e. without a layer of adhesive. Thus, British Patent 2,101,526 discloses the bonding of a polyimide derived from biphenyltetracarboxylic dianhydride directly to metal foil by applying heat and pressure. In other words, the polyimide is formable. It has been found, however, that such formable polyimides have inferior thermal stability to conventional non-formable polyimides.
The object of this invention is to provide flexible polyimide metal-clad laminates, which withstand high temperatures, have good adhesion, good thermal, mechanical and electrical properties and are chemically etchable (for TAB applications).